The present invention relates to a method of reducing syneresis in certain aqueous compositions while, at the same time, increasing the viscosity efficiency of the aqueous system. The method of this invention is particularly applicable to aqueous compositions which contain latex polymers and thickening agents.
Since aqueous based compositions were first introduced, additives have been sought to increase the viscosity and to maintain the viscosity at desired levels under various processing conditions and end use situations. These additives are commonly referred to as xe2x80x9cthickening agentsxe2x80x9d or xe2x80x9cthickenersxe2x80x9d. Thickeners are used in latex paints for viscosity improvement and control as well as for protective colloidal action and for the improvement of pigment suspension, leveling and flow. In addition, the additives often emulsify, disperse and stabilize latex ingredients. In latex paints and textile treating compositions, thickeners often improve the xe2x80x9cstickingxe2x80x9d or binding properties of the overall composition. Thickeners are commonly used in the cosmetic industry as thixotropic agents and to improve softening, sizing and handling properties. As additives to paper coating compositions, thickeners improve thickening under high shear conditions. Thickeners are likewise useful for the foregoing and other properties in oil well drilling and flooding fluids, fire-fighting foams and fluids, detergents, leather pastes and finishes, adhesives, pharmaceuticals, agricultural formulations and emulsions of all types.
Among the many well-known thickeners are natural products such as the alginates, casein, gum karaya, locust bean gum and gum tragacanth, and modified natural products such as the cellulosics, including methyl cellulose, hydroxyethyl cellulose and hydroxypropyl cellulose.
Many synthetic thickeners are also available, such as carboxy vinyl ether copolymers, acrylic polymers and maleic anhydride styrene copolymers. One of the most significant classes of synthetic thickeners available today are comprised of urethane polymers. More specifically, these polymers contain hydrophobic moieties attached to a poly-ethylene oxide backbone. The various polymeric chains are linked together via urethane linkages. This class of thickeners is often referred to as HEUR modifiers, which stands for xe2x80x9chydrophobically modified ethylene oxide urethanexe2x80x9d. The hydrophobic groups are positioned on the ends of the polymeric chain and together contain at least 20 carbon atoms. Since each end of the polymer chain contains a hydrophobic group, these modifiers are referred to as xe2x80x9cdiphobesxe2x80x9d. The polymeric backbone is comprised of hydrophilic polyether segments. Such HEUR modifiers are described in detail in U.S. Pat. Nos. 4,079,028 and 4,426,485.
The HEUR thickeners are referred to as xe2x80x9cassociative thickenersxe2x80x9d because the mechanism by which they thicken involves forming hydrophobic associations between the hydrophobic moieties in the thickener molecules and other hydrophobic surfaces, including other thickener molecules, pigments or latex particles. It is this interaction with latex particles that is the focus of this invention.
Certain formulations containing HEUR modifiers may cause bridging between the latex particles, which, in turn, leads to flocculation or xe2x80x9csyneresisxe2x80x9d of the aqueous latex system. Syneresis is the separation of a liquid from a gel, and manifests itself as an exudation or xe2x80x9csqueezing outxe2x80x9d of diluent from a separate phase. Obviously, such separation is unacceptable when found in aqueous latex paint formulations. However, attempting to solve this problem by removing or reducing the amount of HEUR modifiers results in an unacceptable drop in the viscosity of the formulation. Specifically, an unacceptable drop in mid-shear (Krebs-Stormer) viscosity results, measured as a drop in KU, or xe2x80x9cKrebs Unitsxe2x80x9d.
The present invention solves the problem of syneresis in aqueous latex formulations. Surprisingly, the method of the present invention also results in an increase in the KU efficiency. KU efficiency is defined herein as a measure of the amount of thickener required to achieve a target viscosity. That is, the greater the amount of thickener required to reach a specified viscosity, the lower the KU efficiency. Achieving greater KU efficiency while, at the same time, reducing or eliminating syneresis often involves employing solutions from two diametrically opposed process parameters. To achieve both objectives through a single process step is both surprising and contra-indicated based on the known art.
The invention is directed toward a method of reducing syneresis and increasing the viscosity efficiency in aqueous compositions which contains urethane based, or xe2x80x9cHEURxe2x80x9d, thickeners by adding to the composition a high molecular weight monophobe or a monophobe in combination with a multiphobe. The monophobe contains at least one hydrophilic segment and only one hydrophobic segment. The hydrophobe segment will ideally have a carbon content of more than 12, and preferably in the range of 12 to 18. The monophobes of the invention may be nonionic, anionic, cationic or amphoteric. The Mn (number average molecular weight) of the monophobe hydrophilic segment is at least 2,000, and preferably greater than 5,000, up to 10,000.
The monophobe may optionally contain multiphobe molecules of various chain lengths or molecular weights. A multiphobe is a molecule containing at least one hydrophilic segment and at least two hydrophobic segments. The multiphobe is preferably a diphobe (2 hydrophobic segments). The Mn of the diphobe hydrophilic segment is less than twice the Mn of the monophobe. The hydrophobicity of the diphobe hydrophobic segment is sufficient to form non-specific hydrophobic associations. Preferably, the diphobe hydrophobic segments each contain a hydrocarbon moiety having at least 8 carbon atoms or its equivalent.
In order to achieve the dual objectives of increasing the viscosity efficiency and reducing syneresis, it has been discovered that the multiphobe may be combined with the monophobe. Up to 66% of the total weight of the monophobe plus multiphobe may be multiphobe. If multiphobe is added in excess of this amount, the KU efficiency increases but syneresis is very likely to result.
The monophobe or monophobe-multiphobe mixtures of the invention are added to the aqueous composition as a function of the amount of HEUR thickener present therein. The mixtures of the invention are added in an amount of at least 1% by weight of the HEUR thickener. Preferably, this amount is 2-50% and most preferably, 10-30%.